This paper presents the detection and quantification of hogwash oil in soybean oils by means of absorption spectroscopy. Three types of soybean oils were adulterated with different concentrations of hogwash oil. The spectra were measured in the visible band using a white LED and a low-cost spectrometer. The measured spectra were processed by means of multivariate analysis to distinguish the adulteration and, for each soybean oil, to quantify the adulterant concentration. Then the visible spectra were sliced into two bands for modeling a simple setup made of two LEDs only. The successful results indicate potentials for implementing a smartphone-compatible device for self-assessment of soybean oil quality.